Vacuum processing is commonly used in the manufacture of semiconductor devices to deposit thin films on to substrates. Typically, a processing enclosure is evacuated to a very low pressure, which, depending on the type of process, may be as low as 10−6 mbar, and feed gases are introduced to the evacuated enclosure to cause the desired material to be deposited on one or more substrates located in the enclosure. Upon completion of the deposition, the substrate is removed from the enclosure and another substrate is inserted for repetition of the deposition process.
Significant vacuum pumping time is required to evacuate the processing enclosure to the required pressure. Therefore, in order to maintain the pressure in the enclosure at or around the required level when changing substrates, transfer enclosures and load lock enclosures are typically used. The capacity of the load lock enclosure can range from just a few litres to several thousand litres for some of the larger flat panel display tools.
The load lock enclosure typically has a first window, which can be selectively opened to allow substrates to be transferred between the load lock enclosure and the transfer enclosure, and a second window, which can be selectively opened to the atmosphere to allow substrates to be inserted into and removed from the load lock enclosure. In use, the processing enclosure is maintained at the desired vacuum by a processing enclosure vacuum pumping arrangement. With the first window closed, the second window is opened to the atmosphere to allow the substrate to be inserted into the load lock enclosure. The second window is then closed, and, using a load lock vacuum pumping arrangement, the load lock enclosure is evacuated until the load lock enclosure is at substantially the same pressure as the transfer enclosure, typically around 0.1 mbar. The first window is then opened to allow the substrate to be transferred to the transfer enclosure. The transfer enclosure is then evacuated to a pressure at substantially the same pressure as the processing enclosure, whereupon the substrate is transferred to the processing enclosure.
When vacuum processing has been completed, the processed substrate is transferred back to the load lock enclosure. With the first window closed to maintain the vacuum in the transfer enclosure, the pressure in the load lock enclosure is brought up to atmospheric pressure by allowing a non-reactive gas, such as air or nitrogen, to flow into the load lock enclosure. When the pressure in the load lock enclosure is at or near atmospheric pressure, the second window is opened to allow the processed substrate to be removed. Thus, for a load lock enclosure, a repeating cycle of evacuation from atmosphere to a medium vacuum (around 0.1 mbar) is required.
In order to increase throughput and consequently output of the finished product, it is desirable to reduce the pressure in the load lock enclosure as rapidly as possible. In some systems, such as that described in JP11-230034 and as represented in FIG. 1, this desire has lead to implementation of a pre-evacuated auxiliary chamber 4 acting in combination with a pumping arrangement 3 to evacuate a load lock enclosure 1. The auxiliary chamber 4, which may be isolated from the load lock pumping arrangement 3 by isolation valve 5, is used to initiate the pump down process and assist in achieving improved pump down cycle time. In the illustrated system, the pumping arrangement 3 comprises two booster pumps 6 upstream of four backing pumps 7.
In this system, with isolation valve 2 in a closed position and isolation valve 5 in an open position, the auxiliary chamber 4 is evacuated by the pumping arrangement 3 before evacuation of the load lock enclosure 1 is initiated. When evacuation of the load lock enclosure 1 is required, the isolation valves 2, 5 are both opened so that the load lock enclosure 1 is in fluid communication both with the pumping arrangement 3 and the evacuated auxiliary chamber 4. The pressures within the enclosure 1 and the chamber 4 rapidly equalise, causing a large “slug” of high pressure fluid to rush from the load lock enclosure 1 towards the evacuated auxiliary chamber 4. As the pumping arrangement 3 continues to draw fluid as the pressure equalises between the load lock enclosure 1 and the auxiliary chamber 4, an effect of this slug of high pressure fluid rushing into the auxiliary chamber 4 is a rapid increase in the pressure at the inlets of the booster pumps 6, which causes the rotation speed of the pumping mechanism of the booster pumps 6 to be significantly slowed. For example, the rotational speed of a single stage Roots booster pump will typically vary from a maximum value of approximately 100 Hz when at 0.1 mbar to a lower value of approximately 15 Hz when atmospheric conditions are approached. Consequently, the slug of high pressure fluid experienced by the booster pumps 6 would rapidly reduce the rotational speeds of the booster pumps 6 to approximately 15 Hz.
Once this pressure equalisation has taken place, the auxiliary chamber 4 is isolated from the pumping arrangement 3 by closing isolation valve 5, and further evacuation of the load lock enclosure 1 is carried out by the pumping arrangement 3 alone. As the rotational speed of the booster pumps 6 has been significantly reduced, there is a delay whilst the rotational speed is restored to an appropriate operating level. Indeed, it may take up to 10 seconds to return the booster pumps 6 to their optimum operating conditions of approximately 100 Hz. This delay adds to the overall time to evacuate the load lock enclosure 1.
It is an aim of at least one embodiment of the present invention to reduce the time required to evacuate an enclosure.